Reticle and direct lithography writing strategy

ABSTRACT

The present invention relates to preparation of patterned reticles to be used as masks in the production of semiconductor and other devices. Methods and devices are described utilizing resist and transfer layers over a masking layer on a reticle. The methods and devices produce small feature dimensions in masks and phase shift masks. The methods described for masks are in many cases applicable to the direct writing on other workpieces having similarly small features, such as semiconductor, cryogenic, magnetic and optical microdevices.

This application is a continuation of U.S. patent application Ser. No.09/664,288, filed Sep. 18, 2000.

FIELD OF INVENTION

The present invention relates to preparation of patterned reticles to beused as masks in the production of semiconductor and other devices.Methods and devices are described utilizing resist and transfer layersover a masking layer on a reticle.

RELATED ART

Semiconductor devices include multiple layers of structures. Thestructures are formed in numerous steps, including steps of applyingresist, then exposing, developing and selectively removing the resist toform a pattern of exposed areas. The exposed areas may be etched toremove material or sputtered to add material. A critical part of formingthe pattern in the resist is exposing it. Resist is exposed to an energybeam that changes its chemical properties. One cost-effective way ofexposing the resist is with a stepper. A stepper uses a reticle, whichtypically includes a carefully prepared, transmissive quartz substrateoverlaid by a non-transmissive or masking layer that is patterned withareas to be exposed and areas to be left unexposed. Patterning is anessential step in the preparation of reticles. Reticles are used tomanufacture semiconductor and other devices, such as flat-panel displaysand television or monitor screens.

Semiconductor devices have become progressively smaller. The featuredimensions in semiconductor devices have shrunken by approximately 40percent every three years for more than 30 years. Further shrinkage isanticipated. Current minimum line widths of approximately 0.13 micronswill shrink to 0.025 microns, if the historical rate of developmentcontinues for another 15 years.

The pattern on a reticle used to produce semiconductor devices istypically four times larger than that on the wafer being exposed.Historically, this reduction factor has meant that minimum featuredimensions in the reticles are less critical than the minimum featuredimensions on the surface of the semiconductor. However, the differencein criticality is much less than might be expected and will in the nearfuture disappear.

Critical dimension uniformity, as a percentage of line width, is moreexacting in the pattern on a reticle than in the features on the surfaceof a wafer. On the wafer, critical dimension uniformity of plus or minus10 percent of the line width has historically been acceptable. In theerror budget for the wafer line width, the mask has been allowed tocontribute half of the critical dimension variation, or a variation offive percent of a line width. Other factors use the remaining errorbudget. It has been observed that nonlinearities in transfer of apattern from a reticle to a wafer magnify any size errors in the mask.This is empirically quantified as a mask error enhancement factor (MEEFor MEF). In current technology, the mask error enhancement factor istypically two. Therefore, the critical dimension uniformity on thereticle is reduced to approximately two and one-half percent of a linewidth, to remain within the error budget.

It is anticipated that requirements for critical dimension uniformitywill tighten in time, particularly for masks. On the surface of thewafer, a critical dimension uniformity of plus or minus five percent ofthe line width will be required in the future. At the same time, themask error enhancement factor is likely to increase due to moreaggressive lithographic process trade-offs, such as tuning thelithographic process to optimize the manufacture of contact holes,transistors or other critical features in order to use feature sizescloser to the theoretical resolution limit. For masks, a criticaldimension uniformity of plus or minus one percent of a line width orfeature size is anticipated. At this rate, the tolerance for criticaldimension errors on the mask will be smaller in absolute nanometers thanit is on the surface the wafer, despite the fact that the stepper takesadvantage of a mask that is four times as large as the area on the waferthat is being exposed.

One of the energy beam sources currently used to expose resist is deepultraviolet (DUV), in the wavelength range of 100 to 300 nanometers.This energy source is used with two types of resist to produce masks:conventional positive, so called Novolac-DNQ, resist and chemicallyamplified resist. Essentially all DUV exposure in steppers useschemically amplified resist. The requirements in pattern generators forpatterning of reticles are so different than in steppers that chemicallyamplified resists are unsuitable for patterning reticles. Work to modifyconventional Novolac-DNQ resist to produce a resist suitable for DUVexposure of mask patterns reportedly has failed.

Uniformity and feature size requirements have become so demanding thatwet etching no longer is suitable. Wet etching is generally not useablewhen the size of features approach the thickness of the films thefeatures are etched from. A wet etch etches sideways as much as itetches vertically. Deterioration of the three-dimensional shape of smallfeatures results. When chrome is wet etched with resist as an etch mask,the etchant removes chrome under the resist, referred to asundercutting. Clear areas produced by wet etching chrome with a resistmask typically come out 0.2 microns too large. A wet etched resist imagewith alternating lines and spaces equally 0.4 microns wide, produces achrome mask pattern where the spaces (clear) are 0.6 microns wide andthe lines (dark) are 0.2 microns. This is a large deviation. It isdifficult to compensate for this deviation by changing the data or thedose. For smaller features, narrow lines will simply disappear.Therefore any pattern with features smaller than 0.5-0.6 microns wideneeds to be produced by dry or plasma etching. The plasma process usedto etch chrome produces vertical “line-of-sight” etchingcharacteristics. The chrome is removed only where it is within the lineof sight from the plasma source; essentially no undercutting results.

Issues Using Positive Non-Amplified Resists

Positive non-amplified resists provide excellent performance in theviolet visible and near UV wavelength ranges. This resist is transparentand has high contrast, giving essentially vertical resist walls and goodprocess latitude. It has good shelf life and mask blanks can beprecoated with resist at the time of manufacturing, shipped to users,and kept in storage until needed. Although there is a small decay of thelatent image, plates can in principle be exposed today and developedafter weeks.

In the DUV wavelength range, both the Novolac resin and the photoactivecompound used in Novolac absorb strongly. The edge wall angle afterdevelopment is partly controlled by the absorption of light and partlyby the resist contrast. With high absorption, the features will havestrongly sloping edge walls, whatever the chemical contrast. Nonon-amplified resist formulation is known which combines good contrastwith high transparency.

The effect of non-vertical trench walls is significant for narrow lines.One reason for non-vertical trench walls is that a resist layer iseroded by the plasma during the etching. The uniformity of resisterosion is difficult to control since, among other things, it depends onthe pattern to be etched. Erosion makes the clear areas larger andvarying plasma activity from run to run and across the surface of theworkpiece gives a varying CD between masks and within each mask. Thevariation of the resist thickness at the end of the plasma etching stepmay be 50 nm peak-to-valley or more. For a wall angle of 80 degrees,instead of 90 degrees, a 50 nm variation in resist thickness produces avariation in trench width, at the bottom of the trench, of nearly 20 nm,which may translate into an undesirable three-sigma deviation of 20 nm.This erosion problem is exacerbated by the high optical absorption ofnon-chemically amplified resists used with DUV radiation. High opticalabsorption leads to greater development of the resist at the top of thetrench than the bottom, further increasing the variation in line width.

Resist sidewall deviation from 90° vertical inevitably limits the lineresolution. In 0.5 micron thick resist layer with a side wall angle of80 degrees, a line having a width of 0.025 microns at the top of theresist layer is only 0.2 microns wide at the bottom of the resist layerwhere the chrome is etched. Using current chemistry it is not possibleto make the resist thinner than 0.4-0.5 microns and still protect thechrome during the dry etching. If the line at the top of the resistlayer is narrowed, the wall angle less favorable or the resist thicker,the line would tend to vanish.

Obviously, each of the problems described gets worse as line widths getsmaller, tolerances diminish, and the wavelength move into the deepultraviolet.

Issues with Chemically Amplified Resists

The use of chemically amplified resists introduces other problems.Chemically amplified resists developed for stepper processing aretransparent and have high contrast, giving almost perfectly verticalresist walls. However, they need a thermal annealing or activation stepafter exposure, that is, a post-exposure bake (PEB). Activation andchemical amplification are highly sensitive to the temperature in timeof this bake. Use of chemically amplified resists on reticles is muchmore difficult than on wafers, due to the thickness and shape ofreticles. Reticles are much thicker and less thermally conductive thansilicon wafers, making it more difficult to control the baking sequenceaccurately. Furthermore, reticles are square, leading to corner effectsthat are not experienced with round wafers. These post-exposure bakeproblems are not necessarily limited to chemically amplified resists,but are particularly had for chemically amplified resists due to thecriticality of the baking step. Non-chemically amplified resists aresometimes baked after exposure to even out standing wave interferenceeffects, leading to the same problems. Post-exposure baking alsointroduces a latent image diffusion problem, for both kinds of resist,but worse for the chemically amplified resists since the deactivationpost bake often requires a substantially different temperature than thatoptimized for standing wave reduction.

An additional problem of chemically amplified resists is theirinstability and short working life. Chemically amplified resists havebeen developed for use with steppers, which can finish 100-500 wafers inthe same time that a mask writer produces one mask. Chemically amplifiedresists are spun on the surface of wafers and prebaked shortly beforethey are placed in the stepper and are baked shortly thereafter on anautomated line, within the relatively short working life of the resist.This makes the current generation of chemically amplified resistsunsuitable for use in a mask writer, which may take one to ten hours ormore to write a mask and typically operates without an automatedprocessing line. The related problem is that the time from prebaked topost bake depends on the pattern written and is highly variable. As moresuitable chemically amplified resists are developed for use with maskmaking, it will be necessary to take into account the substantialvariation in mask writing the time.

Issues Common to All Single-Layer Resists

All single layer resists share properties that makes them less suitablein the foreseeable future. The mask pattern is always wet developedsince there exists no process for dry development. Again, the minimumresist layer thickness essentially constant at 0.4-0.5 microns,regardless of the feature size, in order to resist plasma erosion inareas where chrome is not supposed to be removed. As features get verysmall, wet developed resist structures assume an unfavorable aspectratio. In a mask pattern with 10 billion features it is highly likelythat some of these high aspect ration features will be damaged byhydrodynamic forces and surface tension during wet processing.

For optical exposure, single layer resists further require a trade-offbetween transparency and interference effects. The thin resist layerneeds to be transparent to be exposed from top to bottom, but thetransparency makes it subject to optical interference that lowers theeffective performance of the resist and increases process variability.Two interference effects are sometimes referred to as standing wave andbulk effects. The standing wave effect results from interference withinthe resist layer between light directed toward the reticle's surface andlight reflected back. The light directed toward the non-transmissive,mirror-like masking layer and the light reflected back from that layerproduce a standing wave where the crests and troughs of the directed andreflected light align. This produces vertical bands of more and lesscompletely exposed resist. When the resist is developed and selectivelyremoved, there is a tendency for the sides of the resulting trench tobend in and out, which is referred to as the standing wave effect. Therelated bulk effect results from interference above the resist layerbetween light reflected off the surface of the resist and lightreflected off the surface of the reticle and back out of the resist.With certain thicknesses of resist, there is destructive interferencebetween the light entering and leaving the resist, allowing a maximumnumber of photons to stay in the resist layer, producing highsensitivity. Variations in the resist bulk or thickness effect thesensitivity of the film and lead to non uniformity is in the patternproduced. As a resist film is made more transparent, interferenceeffects are reduced, but the etch slope would be worse. These problemsare common to ordinary and chemically amplified resists. In waferlithography the dilemma is normally solved with a thin anti reflectingcoating under the resist and sometimes also on top of the resist aswell.

Mask production faces additional issues. For instance, productioncontrol is difficult due to low production volume. Monitoring andfeedback techniques used to improve the quality of semiconductorproduction are not readily applied to low volume production. Thus, amask shop needs a more stable process through a semiconductor fab.

Thus, it is desirable to develop a new process for patterning reticlesand forming phase shift windows in reticles. The new process preferablywould be suitable for non-chemically amplified resists or yet to bedeveloped amplified resists and would yield very small feature sizeswith great uniformity by avoiding interference effects and other processhazards.

SUMMARY OF THE INVENTION

An objective of the invention is to produce small features on a reticlewith precise critical dimensions, using a technique suitable to avariety of energy sources.

One embodiment of the present invention includes a method of creating apatterned reticle, including creating a latent image in a resist layerusing a pattern generator, creating a plasma etch barrier correspondingto said latent image, directionally etching the transfer layer throughsaid plasma etch barrier, and removing the transfer layer to exposeunetched portions of the masking layer. According to this embodiment,the resist layer maybe wet developed. It may be less than 200 nm thickand preferably 150 nm thick. The transfer layer maybe between 200 and500 nm thick, and preferably 350 nm thick. The plasma etch barrier maycomprise silicon in the resist layer, which may be present before thelatent image is created or may be added after it is created.Alternatively, the plasma etch barrier a comprise a separate filmbetween the resist and transfer layers, preferably deposited bysputtering. This etch barrier film may be a metal containing film,comprising aluminum, a metal oxide, silicon, or silicon oxide. A plasmaetch barrier comprising a separate film may be patterned by plasmaetching through the resist layer. A further aspect of this embodiment isthat the transfer layer maybe essentially non-transmissive to an energybeam used to create the latent image. This transfer layer may be removedusing a first plasma chemistry. The first plasma chemistry may containhalogen ions and maybe an oxygen plasma. The transfer layer maybe anorganic material. Directional etching of the transfer and masking layermay be carried out by RIE type etching and the transfer layer may beremoved by non-preferential oxygen plasma.

Additional embodiment of the present invention includes creatingfeatures on a mask blank, including the steps of exposing a resist layerusing a pattern generator, developing the resist layer and selectivelyremoving portions thereof, directionally etching a transfer layerunderneath the resist layer, directionally etching a masking layerunderneath the transfer layer, and removing the transfer layer to exposeunhedged portions of the masking layer. The pattern generator uses mayuse photon energy, electron beams, or particle beams. When photon energyis used, the transfer layer maybe essentially non-transmissive to thewavelength of photon energy used. A variety of wavelengths can be usedto create a variety of minimum feature dimensions, because there is acritical relationship between wavelength and resulting feature size.Energy of 300 to 380 nm wavelength can be used to create minimum featuredimensions and 75 to 285 nm. Energy of 200 to 300 nm can be used tocreate minimum feature dimensions of 55 to 225 nm. Energy of 100 to 220nm can be used to create minimum feature dimensions of 32 to 124 nm.Energy of five to the 13 nm can be used to create minimum featuredimensions of 6 2 44 nm. When the electron beam is used, less than 3000eV energy is preferred. Minimum feature dimensions of 2270 nm to becreated. Depending on type of energy beam used, the minimum featuredimensions created may be in the ranges of 75-285 nm, 55-225 nm, 32-124nm, or 6-44 nm. An aspect of this embodiment is that the patterngenerator can be aligned to the reticle when the resist and transferlayers are transmissive to a certain, non-exposing wavelength of lightby observing features beneath the resist and transfer layers. With atransfer layer that is more absorptive than the resist layer to anothercertain wavelength of light, the pattern generator can autofocus on theinterface between the resist and transfer layers.

According to a further aspect of the invention, multiple passes may beused to expose the resist layer, preferably four passes. The exposurepasses showed take place in essentially opposing directions, yielding anaverage time between exposure and completion of the final exposurepasses which is essentially equal for locations dispersed across thereticle.

Additional aspect of the present invention is that a plasma comprisingoxygen and silicon dioxide can be used to selectively remove a siliconcontaining resist. The resist can be treated with silicon prior todeveloping. Useful silicon treating compounds includes silane, liquidcompounds and gaseous compounds. The silicon can be treated afterdevelopment and before removal of the resist. Resist development can becarried out by wet or dry development.

Either embodiment of the present invention can be enhanced by aincluding in steps of inspecting and repairing the selectively removedresist. Alternatively, the developed resist can be inspected andfeatures precisely widened to match a critical tolerance.

The directional etching of the transfer or masking layers according toeither embodiment can be carried out by plasma etching or reactive ionetching. Chlorine may be used in etching gas to remove a masking layer.The transfer layer may comprise an organic material, preferably oneadapted to planarizing the masking layer and dyed with a DUV-absorbingdye. The masking layer may comprise more than one physical layer, forinstance a layer of chrome overlaid by an antireflective layer ofnonstoichiometric chrome oxide.

Another embodiment of present invention is a method of preparing areticle blank for patterning, including the steps of forming a maskinglayer a reticle substrate, spinning an organic layer over the maskingthe layer, baking the organic layer, spinning a positivesilicon-containing resist layer over the organic layer, and baking theresist layer. According to this embodiment, the masking layer may becomprised of chrome in the range of 40-90 nm thick. Alternatively, itmay comprise aluminum or tungsten. On a quartz reticle substrate, themasking layer may comprise a patterned structure. The resist layer maybebetween 50 and 200 nm thick, preferably 150 nm thick. The resist andtransfer layers may have different characteristics of absorbing certainwavelengths of light, so that a pattern generator can focus on theinterface between these layers.

BRIEF DESCRIPTION OF FIGURES

FIGS. 1A-1E depict a dual layer reticle blank structure and a processfor preparing a patterned reticle using dual layers of resist andtransfer medium.

FIGS. 2A-2E also depict a dual layer reticle blank structure and aprocess. This process is for preparing a phase shift mask.

FIG. 3 depicts the process of over-etching regions of a phase shiftmask.

FIG. 4 is a block diagram of a reticle with dual layers of resist andtransfer medium.

FIG. 5 is a graph of minimum line width ranges for varying wavelengthsof photon energy beams.

DETAILED DESCRIPTION

Following detailed description of the invention and embodimentspracticing the invention is made with respect to the figures. It ispresented for purposes of illustration and description. It is notintended to limit the invention to the precise forms disclosed. Manymodifications and equivalent arrangements will be apparent to persons ofordinary skill in the art.

FIGS. 1A through 1E depict a coated reticle blank and process steps fora method of removing a non-transmissive layer from the surface of thereticle. In FIG. 1A, the coated reticle comprises a reticle blank 100, amasking or non-transmissive layer 102, a transfer layer 104, and aresist layer 106. Optionally, it may include a plasma resistive layer105 between the transfer layer 104 and the resist layer 106. The reticleblank 100 often comprises a quartz substrate, a Zerodun™ ceramicsubstrate or an ULE™ glass substrate. One form factor currently used is152 mm by 152 mm by 6.25 mm thick. In one style of mask, the blank istransmissive to an energy beam used during the manufacture ofsemiconductor devices. A mask is formed over the blank to block thepassage of the energy beam in areas where resist on a wafer is intendednot to be exposed. Unmasked portions of the blank allow an energy beamto pass through and form a pattern on the wafer resist. In another styleof mask, the portions of the mask-blank system reflect and absorb theenergy used to expose the resist. This style of mask is used inprojection lithography.

With some energy sources, a phase shift mask can be used. The thicknessof a substrate may be altered either by removing some material from thereticle, for instance by etching, or by adding material, such as adielectric, to selected portions of the mask. A thicker mask transmitslight more slowly. When the passage of light is retarded by half awavelength, destructive interference results between adjacent areaswhere light passes retarded and unretarded through the reticle.

The non-transmissive masking layer 102 of a coated reticle typicallyincludes a chrome masking layer approximately 40-90 nm thick. The chromematerial may be applied by sputtered deposition. Alternatively,aluminum, gold, tungsten, or silicon could be used to form thenon-transmissive, masking layer. Optionally, the non-transmissive layermay also include an anti-reflective layer. Non-stoichiometric chromiumoxide material approximately 30 microns thick can be used to reducereflectivity. This enhances performance when the mask is used in astepper, but it is not necessary for patterning the mask. Its presencemay reduce standing wave and bulk interference effects. However, opticalfocusing systems used in both steppers and pattern generating equipmentrequire some reflection to be effective. Therefore, the anti-reflectivecomponent of a non-transmissive layer cannot be perfectly absorbing.Alternatively, the non-transmissive layer 102 could be a structure, suchas used in a so called “chromeless phase shifting” mask, formed on or inthe surface of the reticle which reflects, diffuses or absorbs theenergy beam, so that an energy beam directed to a non-transmissiveregion would not produce a threshold exposure in the resist underlyingthe region. Alignment marks or features may be formed in thenon-transmissive layer, which are useful for aligning the coordinatesystem of the mask making equipment. For a projection style reticle, adifferent type of masking layer is used, which is known in the art, toproduce areas of the reticle which reflect and absorb energy.

Over the non-transmissive layer, but not necessarily directly on it, atransfer layer 104 is applied. This layer may be spun on usingconventional techniques to form a layer approximately 0.2-0-5 micronsthick. Preferably, an organic material is used which includes aDUV-absorbing dye, preferably using a dye selective to absorption ofexposure radiation and transparent to alignment radiation. The materialused in the transfer layer should tend to planarize the surface,especially when spun on. This is particularly useful when thenon-transmissive layer has already been patterned. When a photon beam isused to expose the resist, use of an energy absorbing dye has severaladvantages. For focusing, it provides a target. At the same time, aselectively DUV-absorbing dye would allow an optical alignment systemworking at 532 nm to take advantage of the transparency of the transferlayer and overlaying resist layer to align two features in thenon-transmissive layer, particularly after initial patterning of alayer. The transfer layer reduces the amount of light reflected from thenon-transmissive layer, thereby minimizing standing wave effects. At thesame time, it reduces the amount of reflected light which escapes thetransfer layer, thereby minimizing bulk effects. The transfer layerproduces these advantages without the disadvantage of higher absorptionon top than at the bottom of the resist layer. After application of thetransfer layer, baking at 150-180 degrees Celsius will drive out thesolvent and improve the resistance to the transfer layer to plasmaetching.

Optionally, over the transfer layer, but not necessarily directly on it,a plasma resistive layer 105 is applied. This layer may be sputtered on.It preferably is a silicon layer, which forms silicon dioxide whenexposed to certain plasmas, particularly an inorganic silicon layer.

Over the transfer layer and the optional plasma resistive, but notnecessarily directly on them, a resist layer 106 is applied. This layermay be spun on using conventional techniques to form a layerapproximately 0.05-0.20 micron thick, and more preferably, approximately0.15 micron thick. Is preferred to use a positive resist, because it iseasier to expose fine lines than to expose the surrounding area, leavingfine lines unexposed. The resists used with photon energy may bereferred to as photoresists. Other types of resist are used with otherforms of energy. Optionally, a silicon-containing resists may be used toenhance the selectivity of plasma etching, as described further below.The silicon content of approximately 7 to 10 percent is desirable. Afterapplication of the resist layer, baking at 90 degrees Celsius will driveout the solvent. The material selected for the transfer and resistlayers preferably should have good shelf life and stability so that themask-blank manufacturer can precoat the blanks.

The refractive index of the resist layer 106 should match as closely aspractical the refractive index of the transfer layer 104, in order tominimize standing wave interference effects. The standing waveinterference effects of the double layer system, using resist over atransfer layer, can be analyzed by applying Brunner's formula. Theaction of bottom and top antireflective layers on the swing amplitude Sis described in good approximation by Brunner's formula, when a twolayer system is treated as a single layer:

S=4{square root over (R _(b) ·R _(t))}·e ^(−αd)

Where, R_(b) is the reflectivity at the interface between the transferlayer and the masking layer. R_(t) is the reflectivity of the interfacebetween the resist and air, which may be reduced by an antireflectivetop coating. “alpha” is the rate of absorption of exposure radiation bythe resist, per unit thickness. “d” is the thickness of the absorbingresist layer. When the resist and transfer layers have approximately thesame refractive index, the reflection at that interface approaches 0.Thus, Brunner's formula can be applied to the resist and transfer layersas a single layer. The combined single layer has a value of alpha*d>>1,so the exponential factor in Brunner's formula is very small. As theexponential factor vanishes, the value of S for the combined resist andtransfer layers is also very small. The exact value of alpha*d can bechosen by varying the dye content of the organic material or transferlayer. The two layer system allows much greater freedom in adjusting thevalue of alpha*d than a single resist layer, where a low absorbancevalue of alpha*d<1 is ordinarily preferred. For a single layer resist,the low absorbance is ordinarily preferred to give an acceptable edgeslope, so that the sides of the trench after development and selectiveremoval of portions of the resist will be nearly vertical.

A variation on FIG. 1A appears in FIG. 2A. The structure includes atransmissive substrate 200, non-transmissive layer 202, transfer layer204 and resist layer 206. In this structure, part of thenon-transmissive layer 202 was selectively removed before additionallayers were applied. It is particularly useful for the transfer layer inthis structure to be suitable for planarizing, because thenon-transmissive layer is patterned. As patterning is performed at amask shop, a structure depicted in FIG. 2A is likely to be formed at amask shop and unlikely to be stored for long periods of time.

A process embodying the present invention is depicted in FIGS. 1Athrough 1D, with a process variation shown in FIG. 1E. In FIG. 1A, theresist layer 106 is exposed to an energy beam 108. In practice, thisradiation or energy may be any of wide variety of types. Photon energymay be in the UV, the DUV, EUV or x-ray spectrum ranges. For instance,photon energy may be a spectrographically separated or processed througha cut filter from a high-pressure mercury vapor arc light or super highpressure xenon-mercury light, at the g line (approximately 436 nm), theh line (approximately 406 nm), the i line (approximately 365 nm) or thej line (approximately 313 nm). Photon energy also may be generated by ahelium cadmium source (approximately 442 and 325 nm), a solid statesource (approximately 430 and 266 nm) a krypton ion source(approximately 413 nm), an argon ion source (approximately 364 and 257nm). Or, it can be generated by an excimer source or a krypton-fluorideor an argon-fluoride laser (approximately 308, 248, 193, 157 or 126 nm).The NanoStructures Laboratory of the Massachusetts Institute ofTechnology has additionally identified an undulator light source at theUniversity of Wisconsin (approximately 13 nm) and the L line, of copper(approximately 1.32 nm) from a helium-filled exposure chamber as sourcesused in research. Other wavelengths produced by a xenon gas capillarydischarge take include 13.5 nm and 11.4 nm. An electron bombardmentsource yields 4.5 nm radiation. The wavelengths of these photon energysources are critical to the minimum feature dimensions that may becreated, with shorter wavelengths being more difficult to use and havingmore potential to generate smaller features.

Likely feature sizes for many photon energy sources are illustrated inthe table below:

Widest Narrowest Source HeCd 442 497 249 221 111  Solid state 430 484242 215 108  Kr-ion 413 465 232 207 103  Ar-ion 364 410 205 182 91 HeCd325 366 183 163 81 Excimer 308 347 173 154 77 solid statex4 266 299 150133 67 Ar-ionx2 257 289 145 129 64 Excimer 248 279 140 124 62 Excimer193 217 109  97 48 Excimer 157 177  88  79 39 Excimer 126 142  71  63 32 13  29  15  13  7  11  25  12  11  6  5  11  6  5  3

These values are calculated based on k₁=0.45, 0.20 and NA=0.20, 0.40,0.80.

The critical relationship between wavelength and linewidth isillustrated in FIG. 5. This relationship is expressed as:${{M\quad L\quad W} = {k_{1} \cdot \frac{\lambda}{N\quad A}}},$

where MLW is the minimum line width, k₁ is an empirical factor, which ismore favorable when optical proximity correction measures areimplemented, λ is the wavelength of the photon source, and NA is thenumerical aperture for exposure.

The relationship in FIG. 5 depicts a 3 to 1 range of MLW, with thenarrowest lines based on k₁=0.20 and NA=0.80. The widest lines reflectless favorable values of k₁, and NA. Accordingly, in a wavelength rangeof 380-450 nm, which brackets the 413-442 nm sources, the criticalminimum feature dimensions or minimum line widths are approximately95-340 nm. In the wavelength range 300-380 nm, which brackets the 308and 364 nm sources, the critical minimum feature dimensions or minimumline widths are 75-285 nm. In the wavelength range of 220-300 nm, whichbrackets the 248 and 266 nm sources, the critical minimum line widthsare 55-225 nm. In the wavelength range of 100-220 nm, which brackets the126-193 nm sources, the critical minimum line widths are 32-124 nm. Inthe wavelength range of 5-13 nm, the minimum line width is based onk₁=0.45, NA=0.40 and a 3 to 1 range of widest to narrowest linesproduced under varying k₁ and NA factors. The critical minimum linewidths for these wavelength sources are 6-44 nm. In this manner, rangesof critical minimum feature dimensions can be matched to individualsource wavelengths, ranges of source wavelengths, or bracketed sourcewavelengths. Alternatively, particular minimum critical dimensions couldbe claimed for each wavelength source from the data in FIG. 5.

In addition to photon energy, low-energy electron beams and chargedparticle beams have been suitably used for exposing resists. The RaithTurnkey 150 system, produced by Raith company in Dortmund, Germany israted for electron beams of 200 eV to 30 KeV. MIT's NanoStructuresLaboratory reports that it can operate at beam energies as low as 10 eV.An ion beam source for writing a pattern on a suitable resist isdescribed by Westererg and Brodie, “Parallel Charged Particle BeamExposure System,” U.S. Pat. No. 4,465,934. Most generally, the energysource being used needs to be matched to the characteristics of theresist being exposed.

Exposure of the resist is performed using a pattern generator. Forphoton energy, a laser pattern generator or an interference lithographysystem may be used. For electrons, an electron-scanning device may beused. Etec, a subsidiary of Applied Materials, sells an ALTA™ line ofscanning laser pattern generators. Micronic Laser Systems of Taby,Sweden sells an Omega™ line of scanning laser systems and has describeda Sigma™ line of micromirror-based systems. The NanoStructuresLaboratory, working with the University of Wisconsin in some aspects,has described interference lithography systems with spatial periods of200 nm, 100 nm, and 50 nm. The 100 nm spatial period system has beenused to create features (reassembling silicon whiskers) having diametersof 13 nm. The NanoStructures Laboratory also has described a Zone-PlateArray lithography system employing micromirrors to generate lines 200 nmin width, with improvements anticipated to generate lines 20 nm inwidth. The Etec subsidiary of Applied Materials also sells a MEBUS™ lineof Gaussian beam pattern generators.

At least in the case of DUV energy, it is preferred to use four passesor more to generate the scanned pattern. Pattern generation should bearranged so that the average energy dose and the average time fromdosing to completion are approximately constant for different points onthe mask. The preferred strategy for doing this is to write in onedirection for some passes and to write in essentially the oppositedirection for other passes. This is conveniently done writing in a firstdirection on one pass and writing in a second, essentially oppositedirection on the subsequent pass. This approach helps control the decayof latent images in the resist layer. At the time the resist isdeveloped, this writing strategy yields approximately equal averagetimes from exposure to development throughout the reticle.

The exposure in FIG. 1A forming a latent image in the resist is followedby developing and selectively removing portions of the resist. Wetdeveloping is suitable, to be followed by rinsing and drying. Thepatterned resist is depicted in FIG. 1B. Some resist 106 remains. Inother places 115, resist has been removed creating trenches. The sidesof the trench 115 are somewhat sloped due to the isotropic action of thedeveloping and selective removal process and to the absorbance patternof light in the resist.

The selective removal of resist is optionally followed by inspection andrepair of the patterned resist layer. In some circumstances inspectionand repair at this stage may be more effective than if it is done later,particularly when these process steps lead to etching of a phase shiftwindow in the reticle substrate, as depicted in FIG. 2. Alternatively,inspection and repair could follow etching of the transfer layer, stillpreceding etching of either the non-transmissive layer or a phase shiftwindow in the patterned mask.

Precise correction the minimum feature dimensions may be accomplished bywriting and developing openings in the resist which are slightly toosmall, such as 10 nm narrower than desired. Inspection tools can be usedto measure very precisely the width of lines written. A slight isotropicetching, for instance by gas, wet etching or plasma, can be employed toadjust the size of openings in the resist layer, widening lines by 5 to15 nm before pattern transfer. This form of correction improves theuniformity of minimum feature dimensions. It also cleans up the patternsin the resist.

A process variation is shown in FIG. 1E, which involves silylation ofthe resist. In some instances it will be preferable for the resist to beinfused with a silicon-containing compound, such as silane. A liquid orgaseous silicon-containing compound 114 is applied over the resist. Thismay be done either after development and selective removal, as depictedin FIG. 1E, or before development. One option is dry development of theresist after silylation of the latent image. Another option, notseparately shown, is to include silicon content in the top of thetransfer later.

When a plasma resistive layer 105 is present, the logical step ofcreating a plasma resistive layer corresponding the latent image in theresist may involve more than one process step. Separate process stepsmay be used to develop and selectively remove the resist and then toremove corresponding areas of the plasma resistive layer. These stepsmay precede or follow correction of minimum feature dimensions in theresist layer.

Returning to FIG. 1B, the patterned resist layer is exposed todirectional etching. The transfer layer below is directionally etched,with a strongly vertical preferential etch gas 110. Techniques fordirectional etching include reactive ion etching (RIE) and plasmaetching. Plasma etching generally takes place in or near gas dischargeusing a low-pressure process gas such as O₂, CF₄, etc. Various forms ofplasma etching can be used for near-isotropic etching or for verticalanisotropic pattern transfer etching. Suitable processed gases andplasma conditions have been developed for etching thin-film materialsused microlithography, such as silicon, silicon dioxide, aluminum,chromium, resist, and polyamide. One reference on suitable process gasesand plasma conditions is Handbook of Plasma Processing Technology, NoysePublications, 1990, ISBN 0-8155-1220-1. Reactive ion etching is suitablefor vertically anisotropic etching with good line width control. Thesimplest configuration for RIE equipment is a parallel plate etcher, inwhich the work piece is placed on the RF-driven electrode, typicallydriven at a frequency of 13.56 MHz. A discharge in the plasma creates aDC bias which accelerates ions towards the surface the work piece. Aplasma pressure of 10 to 50 millitorrs is often used. Other reactortypes such as an inductively coupled plasma reactor also can be used.

A suitable plasma for etching through the transfer layer may includeoxygen and a small amount of sulfur dioxide. Including silicon in aresist 106 forms a silicon dioxide etch barrier which protects thepatterned resist the oxygen plasma, reducing erosion of the resist. Theresult of the process depicted in FIG. 1B is the structure in FIG. 1C. Avertical or near vertical trench 117 cuts through the transfer layer104, after exposure to the etch gas. Some resist 106 and organicmaterial 104 remains.

The patterned resist 106 and transfer layer 104 are subjected to anadditional directional etching gas 112. To etch through thenon-transmissive layer, a slightly different gas mixture is used. Asuitable plasma may contain a halogen, such as chlorine. The compositionand energy of the plasma should be selected so that it removes thetransmissive layer in the exposed trenches 117. It also may be desirablefor this plasma step to remove the silicon-containing resist 106. Thisdirectional etching step may be carried out in the same device orapparatus as is used to transfer the pattern from the resist layer 106to the transfer layer 104. Using the same RIE device, plasma etcher orother apparatus would minimize the number a wafer transfers required topattern the non-transmissive layer. The result of this process toappears as the structure in FIG. 1D. The non-transmissive layer 102 hasbeen etched yielding a trench 119.

The width of the trench 119 is likely to be the minimum feature sizegenerated by this process. This minimum feature size relates thewavelength of the energy used to expose the resist, when photon energyis used. Shorter wavelengths present a number of problems, whichpractically restrict their use to production of very small featuresizes. For instance, EUV energy is not readily focused usingconventional lenses. It is absorbed in glass and passes through manyconventional mirror materials. Given a practical trade-off, EUV energyhaving wavelengths of 5 to 13 nm is most likely to be used to generatefeatures having minimum dimensions of 6 to 44 nm. DUV energy is expectedto be generated by an excimer, gas or solid state source havingwavelengths of approximately 100-220 nm and is most likely to be used togenerate features having minimum dimensions of 32-124 nm. DUV energyhaving wavelengths of 220 to 300 nm is most likely to be used togenerate features having minimum dimensions of 55 to 225 nm. UV energycan be generated by spectrographically separating or processing througha cut filter the emissions of a high-pressure mercury vapor arc light orsuper high pressure xenon-mercury light. This approach produces i lineenergy of approximately 365 nm or j line energy of approximately 313 nm.Alternatively, a helium-cadmium laser may be used to produce radiationapproximately 325 nm. Near UV energy having wavelengths of 300 to 380 nmis most likely used to generate features having minimum dimensions of 75to 285 nm. Other energy beams can be generated by spectrographicallyseparating or processing through a cut filter the emissions of ahigh-pressure mercury vapor arc light or super high pressurexenon-mercury light. This approach produces g line energy ofapproximately 436 nm or h line energy of approximately 406 nm. Otherenergy of 380-450 nm wavelength is most likely used to generate featureshaving minimum dimensions of 95-340 nm. The use of low-energy electronbeams in the practice the present invention has the potential togenerate minimum feature sizes of 10 to 100 nm. Charged particle beamsmay generate minimum feature dimensions of 5 to 50 nm. As described, thewidth of the trench 119 is critically dependent upon the type of energybeam used to expose the resist. The present invention improves thecritical dimension control and ability to produce fine lines acrosstypes of energy beam.

The same plasma reactor used for pattern transfer can also be used forashing to remove the remainder of the transfer and resist layers. Forpattern transfer, the plasma reactor needs to produce a plasma streamfor etching which is as vertically anisotropic as possible. Reactive ionetching or equivalent will accomplish this. The plasma is low-pressure.A high potential is created on the work piece. A flat plate reactor maybe used to generate the high potential on the work piece using RFenergy. The effect is somewhat like sputtering. For ashing to remove thetransfer layer, and an isotropic process is preferred, though verticalalignment of the plasma is not critical. A relatively high-pressure ofplasma is used, exceeding 200 millitorrs. The work piece has a lowpotential. The plasma creates reactive species which diffuse to thesurface and etch the transfer layer chemically, to remove it. A flatplate reactor may be used with the work piece grounded. Alternatively, abarrel reactor with microwave excitation of the plasma may be used. Forashing, oxygen with a small amount of sulfur dioxide is suitable forremoval of organic residues. In the flat plate reactor, a double RFdrive with double matching networks may be particularly useful. With adouble drive, the cathode and work piece table are driven independently.A single crystal oscillator can generate the RF frequency for bothdrives. By controlling the phase and power of the two drivesindependently, the work piece potential can be controlled within widelimits. One alternative to a flat plate reactor is the split cathodedesign described in L. Hollins et al., Journal of ScientificInstruments, Vol. 1, p. 32 (1968).

The process described with respect to FIG. 1 also applies to etching aphase shift window in a reticle, as depicted in FIG. 2. In FIG. 2A, theinitial structure before exposing the resist layer with energy isdepicted. The coated reticle comprises a blank reticle 200, one or morenon-transmissive layers 202, a transfer layer 204 and a resist layer206. The blank reticle 200 typically is a quartz substrate, as describedabove. The non-transmissive layer 202 of the structure has beenpatterned, for instance by using the process described above. Thenon-transmissive layer of the coated reticle typically includes apatterned chrome layer approximately 40-90 nm thick. Alternatively,aluminum, gold, tungsten or silicon can be used to form thenon-transmissive, masking layer. Optionally, the non-transmissive layermay also include an anti-reflective layer. Non-stoechiometric chromiumoxide material approximately 30 microns thick can be used to reducereflectivity. Again, this enhances performance when the mask is used inthe stepper, but it is not necessary for creating phase shift windows inthe mask. Alternatively, the non-transmissive layer 202 could be astructure formed on or in the surface of the reticle which reflects ordiffuses an energy beam, so that the energy beam projected on anon-transmissive region would not produce a threshold exposure in theresist underlying that region. The pattern generated in thenon-transmissive layer is useful for aligning the coordinate system ofthe mask making equipment.

Over the non-transmissive layer, but not necessarily directly on it, atransfer layer 204 is applied. This is relatively thick layer,preferably of organic material. A suitable material is Novolac, theresin component used in most positive non-amplified photoresist. It hasexcellent adhesion and good plasma etch resistance, and is transparentin visible and UV and absorbing in the DUV. Conventional techniques forspinning on this layer can be used to form a layer of approximately 0.2to 0.5 microns thick, and preferably approximately 0.35 microns thick.If the organic material is not inherently absorbing, and is used withoptical wavelengths it can include an absorbing dye, preferably a dyethat selectively absorbs exposure radiation and is relativelytransparent to alignment radiation. It is particularly useful that thetransfer layer material tends to planarize the surface, especially whenspun on. The differential absorbance of the dye permits different energybeam to be used for exposure and alignment, without the alignment energybeam compromising the feature size. The absorbance of exposure radiationminimizes interference effects, both standing wave and bulk interferenceeffects.

Optionally, over the transfer layer, but not necessarily directly on it,a plasma resistive layer 205 is applied. This layer may be sputtered on.It preferably is a silicon layer, which forms silicon dioxide whenexposed to certain plasmas, particularly an inorganic silicon layer.

Over the transfer layer and the optional plasma resistive, but notnecessarily directly on them, a resist layer 206 is applied.Conventional techniques for spinning on this layer may be used to form alayer approximately 0.05 to 0.20 microns thick, and preferably 0.15microns thick. Optionally, a silicon-containing resist may be used toenhance the selectivity of plasma etching. The resist may containsilicon initially, before it is baked, or a silylation process can beused to infuse silicon in the resist. Because the structure depicted inFIG. 2A has a patterned structure, it is anticipated that the transferand resist layers will be applied in the mask making shop, afterpatterning of the non-transmissive layer. A good working life for thetransfer and resist layers is more important than a good shelf life.

A process embodying the present invention, following the sameprogression as in FIG. 1 is depicted in FIGS. 2A through 2D, with aprocess variation shown in FIG. 2E. Not shown in these figures use theuse of an optical alignment system using as 532 nm photon energy beamsource, taking advantage of the transparency of the resist and thetransfer layer at this wavelength to see the pattern in thenon-transmissive layer. Following alignment of the pattern generatorcoordinate system with the patterned reticle, the resist layer 206 inFIG. 2A is exposed to an energy beam. This may be a photon energy beam,a low-energy electron beam, a charged particle beam or any other energybeam suitable for exposing the particular resist being used. The energybeam exposes the resist using a pattern generator. The patterngeneration scheme should use multiple passes so that the average energydose and the average time from dosing to completion is relativelyconstant across the mask. This helps control the decay of latent imagesin the resist layer.

Developing and selectively removing the resist follows the exposure inFIG. 2A. The patterned resist is depicted in FIG. 2B. The trench 215 isover all or part of an area where the non-transmissive layer has beenetched away. In practice, the etched area of the non-transmissive layermay be wider than the trench 215, where a phase shift window is desired,because phase shift windows are often adjacent to non-shift windows inthe non-transmissive layer.

Inspection and repair of the trench 215 optionally follows selectiveremoval of the resist. In some cases, inspection and repair at thisstage may be more effective than it is done later. Repair of thepatterned resist is likely to be easier than modifying the shape of aphase shift window etched into a substrate such as quartz. The structureresulting from directionally etching the patterned resist 206 withplasma 210 is depicted in FIG. 2C.

A process variation is shown in FIG. 2E, which involves silylation ofthe resist after exposure. In some instances it will be preferable forthe resist to be infused with silicon compound, such as silane, afterpatterning. A liquid or gaseous silicon-containing compound 214 isapplied over the resist. This may be done either after development andselective removal, as depicted in FIG. 2E, or it may be done beforedevelopment of the resist. One option, when silylation is performedprior to development, is dry development of the resist.

FIG. 2C depicts a trench 217 through the transfer layer 204 exposing thereticle substrate 200. Additional plasma 212 is used to directionallyetch a phase shift window in the substrate 200, as depicted in FIG. 2D.

To improve uniformity, an additional phase shift, for instance anadditional 180-degree phase shift, may be added to a patterned mask, inaddition to 180-degree phase shift windows previously created. FIG. 3Adepicts a structure in which a 180-degree phase shift window has alreadybeen etched. The substrate 300 is overlaid by one or morenon-transmissive layers 302. A 180-degree phase shift window has beenetched in part of the substrate 321. In one area of the reticle 323, thenon-transmissive layer has been removed but no phase shift window hasbeen etched.

Either directional etching, depicted in FIG. 3B, or isotropic etching,depicted in FIG. 3C, can accomplish an additional phase shift etching.Plasma 310 maybe used for directional etching, choosing plasma suitableto leave the non-transmissive layer intact while removing a portion ofthe substrate. Alternatively, a relatively nondirectional plasma 316 ora wet etch 316 can be used. The resulting structure is depicted in FIG.3D. The phase shift window 325 appears as being etched deeper into thesubstrate than the non-phase shift transmissive window 327.

The basic steps of the process can be expressed in a list:

1. Begin with a reticle having a dual layer coating.

2. Expose the top, resist layer to an energy beam using a patterngenerator to create a latent image.

3. Create a plasma etch barrier, corresponding to the latent image.

4. Directionally etch the transfer layer through the plasma etchbarrier.

5. Remove the transfer layer, exposing the reticle substrate.

A method for preparing a reticle blank to be exposed is illustrated inFIGS. 4A-4C. This method forms a masking layer 402 over, but notnecessarily on the reticle substrate. A transfer layer 404 is formedover, but not necessarily on the masking layer 402. An optional plasmaresistive layer 405 may be formed over, but not necessarily on thetransfer layer 404. A resistive layer 406 may be formed over, but notnecessarily on the transfer and optional plasma resistive layers 404,405.

Methods and devices practicing the present invention yield a variety ofadvantages. The process is relatively insensitive to the thickness ofthe resist and transfer layers, particularly the transfer layer.High-resolution, fine features are readily generated. Undercutting andwidening of clear spaces is avoiding, facilitating generation ofuniformly alternating lines and spaces. The sidewalls of trenches arenearly vertical. It is unnecessary to attempt a post-exposure bake.Without a post-exposure bake, image diffusion is minimized. For theinitial patterning of the non-transmissive layer, precoated blanks canbe used. The system is relatively insensitive to timing delays, therebysimplifying workflow. Use of a thin, transparent resist layer on top ofa relatively thick underlayer or transfer layer having the nearly thesame refractive index and a higher absorption minimizes interferenceeffects, both standing wave and bulk interference effects. The sameplasma reactor may be used for several process steps, to minimize thetransfer of the reticle among pieces of equipment. This minimizescapital expenditure, floor space requirements, handling, and turnaroundtime. Inspection and repair of the resist layer tends to assure that thefinished mask pattern has critical dimensions matching those intendedand required.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

What is claimed is:
 1. A method of exposing a reticle using a patterngenerator, including exposing a resist layer over the reticle in aplurality of exposure passes, said exposure passes made in a firstdirection and a second direction, the first and second directions beingessentially opposed.
 2. The method of claim 1, wherein the plurality ofexposure passes includes at least four exposure passes.
 3. The method ofclaim 1, wherein successive passes are written in opposing directions.4. The method of claim 2, wherein successive passes are written inopposing directions.
 5. The method of claim 2, wherein the exposurepasses utilize a photon energy beam.
 6. The method of claim 2, whereinthe pattern generator utilizes an electron beam.
 7. The method of claim2, wherein the pattern generator utilizes a Gaussian energy beam.
 8. Themethod of claim 2, wherein the pattern generator utilizes a particlebeam.
 9. A method exposing a reticle using a pattern generator,including exposing a resist layer over the reticle in a plurality ofexposure passes, said exposure passes having an average time fromexposure to completion of all exposure passes that is essentially equalfor locations disbursed across the reticle.
 10. The method of claim 9,wherein the plurality of exposure passes includes at least four exposurepasses.
 11. The method of claim 9, wherein successive passes are writtenin essentially opposing directions.
 12. The method of claim 10, whereinsuccessive passes are written in opposing directions.
 13. The method ofclaim 10, wherein the pattern generator utilizes a photon energy beam.14. The method of claim 10, wherein the pattern generator utilizes anelectron beam.
 15. The method of claim 10, wherein the pattern generatorutilizes a Gaussian energy beam.
 16. The method of claim 10, wherein thepattern generator utilizes a particle beam.
 17. A method exposing aresist layer, including exposing the resist layer in a plurality ofexposure passes, said exposure passes made in a first direction and asecond direction, the first and second directions being essentiallyopposed.
 18. The method of claim 17, wherein the plurality of exposurepasses includes at least four exposure passes.
 19. A method exposing aresist layer, including exposing the resist layer in a plurality ofexposure passes, said exposure passes having an average time fromexposure to completion of all exposure passes that is essentially equalfor locations disbursed across the resist layer.
 20. The method of claim19, wherein the plurality of exposure passes includes at least fourexposure passes.
 21. The method of claim 19, wherein successive passesare written in essentially opposing directions.
 22. The method of claim20, wherein successive passes are written in opposing directions.